Expandable interbody fusion device with graft chambers

ABSTRACT

An expandable interbody fusion device includes superior and inferior endplates that are configured to receive a sequentially inserted stack of expansion members or wafers in interlocking engagement. The expansion members are formed to each have a generally U-shaped rearward facing opening. The superior and inferior endplates have openings through their outer surfaces in at least partial alignment and communication with the rearward facing openings of the expansion members. The inferior endplate has a fully bounded cavity for telescoping receipt of the superior endplate. The inferior endplate also has a fully bounded channel extending through the rear endwall thereof in direct communication with the rearward facing opening of at least one expansion member for the receipt of bone graft material into the device to promote fusion between opposing vertebral bodies of the spine.

FIELD OF THE INVENTION

The subject invention relates generally to the field of spinal implantsand more particularly to expandable interbody fusion devices with graftchambers.

BACKGROUND OF THE INVENTION

Spinal implants such as interbody fusion devices are used to treatdegenerative disc disease and other damages or defects in the spinaldisc between adjacent vertebrae. The disc may be herniated or sufferingfrom a variety of degenerative conditions, such that the anatomicalfunction of the spinal disc is disrupted. Most prevalent surgicaltreatment for these conditions is to fuse the two vertebrae surroundingthe affected disc. In most cases, the entire disc will be removed,except for a portion of the annulus, by way of a discectomy procedure. Aspinal fusion device is then introduced into the intradiscal space andsuitable bone graft or bone substitute material is placed substantiallyin and/or adjacent the device in order to promote fusion between twoadjacent vertebrae.

Certain spinal devices for achieving fusion are also expandable so as tocorrect disc height between the adjacent vertebrae. Examples ofexpandable interbody fusion devices are described in U.S. Pat. No.6,595,998 entitled “Tissue Distraction Device”, which issued on Jul. 22,2003 (the '998 patent), U.S. Pat. No. 7,931,688 entitled “ExpandableInterbody Fusion Device”, which issued on Apr. 26, 2011 (the '688patent), and U.S. Pat. No. 7,967,867 entitled “Expandable InterbodyFusion Device”, which issued on Jun. 28, 2011 (the '867 patent). The'998 patent, the '688 patent and the '867 patent each disclosessequentially introducing in situ a series of elongate inserts referredto as wafers in a percutaneous approach to incrementally distractopposing vertebral bodies to stabilize the spine and correct spinalheight, the wafers including features that allow adjacent wafers tointerlock in multiple degrees of freedom. The '998 patent, the '688patent and the '867 patent are assigned to the same assignee as thepresent invention, the disclosures of these patents being incorporatedherein by reference in their entirety.

Certain interbody fusion devices also include hollow portions orchambers that are filled with suitable material such as bone graft topromote fusion between vertebral bodies. The extent and size of thechambers establish areas of contact that are configured so as to assuremaximum contact between the bone graft and the vertebral bodies.Sufficient surface area of the device surrounding the chambers needs tobe maintained in order to provide an appropriate load bearing surface towithstand the compressive forces exerted by the opposing vertebralbodies. In addition, where expandable interbody fusion devices are usedto correct height within the intradiscal space, the effect of shearforces on the expanded device due to torsional movement of the spinealso needs to be considered.

Accordingly, there is a need to develop expandable interbody fusiondevices with bone graft chambers that take into account and balancethese factors, as well as to facilitate the introduction of bone graftinto the device and through the graft chambers once expanded.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved expandabledevice with openings serving as bone graft chambers for implantationinto the intradiscal space between two opposing vertebral bodies of aspine having the facility for introducing bone graft thereinto uponexpansion.

DESCRIPTION OF THE FIGURES

FIG. 1 is front perspective view of an expandable interbody fusiondevice in unexpanded condition in accordance with one embodiment of thepresent invention.

FIG. 2 is a perspective cross sectional view of the unexpanded device ofFIG. 1 as seen along viewing lines II-II of FIG. 1.

FIG. 3 is a rear perspective view of the device of FIG. 1.

FIG. 4 is a top perspective view of an interlocking wafer serving as anexpansion member to expand the interbody fusion device of FIG. 1.

FIG. 5 is a bottom perspective view of the interlocking wafer shown inFIG. 4.

FIG. 6 is front perspective view of the expandable interbody fusiondevice FIG. 1 expanded to an expanded condition.

FIG. 7 is a perspective cross sectional view of the expanded device ofFIG. 6 is seen along viewing lines VI-VI of FIG. 6.

FIG. 8 is a top perspective view of an inserter for inserting wafersreleasably connected to the unexpanded device of FIG. 1.

FIG. 9 is longitudinal cross sectional view of the inserter of FIG. 8.

FIG. 10 is a perspective view of the guide used with the inserter ofFIG. 8 releasably connected to the expanded device of FIG. 6.

FIG. 11 is a top perspective view of an alternative lordotic expandablefusion device.

DESCRIPTION OF THE EMBODIMENTS

For the purposes of promoting and understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

In accordance with one embodiment of the invention, an expandableinterbody fusion device 10 includes a first superior endplate 12 and asecond inferior endplate 14, as shown in FIGS. 1-3. The interbody fusiondevice 10 has a height across the superior and inferior endplates 12, 14in the unexpanded condition as illustrated in FIGS. 1-3 that is lessthan the normal anatomic height of a typical intradiscal space. Theinvention contemplates that a series of expansion members, such asinterlocking wafers 100 as will be described, are introduced into thedevice 10 to distract the opposing vertebrae by separating the superiorand inferior endplates 12, 14 in situ. Insertion of the wafers 100separates the endplates 12, 14 to expand the height of the device withinthe intradiscal space and to ultimately restore the normal anatomicheight of the disc space. Expansion devices of this type are shown anddescribed in the '998 patent, the '688 patent and the '867 patentdescribed hereinabove and incorporated herein by reference.

The present invention contemplates an improved interbody fusion device10 that particularly includes openings that define graft chambers forcontainment of materials that promote bone fusion through the devicebetween opposing vertebral bodies.

The superior endplate 12 as shown in FIGS. 1-3 and 6-7 is elongate andcomprises a hub 16 having pair of side surfaces 18 and 20 extendinglongitudinally on each side of the hub 16 and a pair of end surfaces 22and 24 extending respectively at the proximal rear end and the distalfront end of the superior endplate 12. The hub 16 is sized andconfigured to fit within a cavity of the inferior endplate 14 fortelescoping movement therewithin, as will be described. The lowersurface 26 of the hub 16 (FIG. 2) includes a shaped configurationdefined by wafer mating features 28 that are substantially identical tothe mating features on the lower surface of each wafer 100, as will bedescribed. The hub 16 defines a series of grooves 30 as shown in FIG. 6extending along each side surface 18 and 20 thereof that is configuredto engage ribs (not shown) projecting interiorly of the inferiorendplate 14. This engagement temporarily holds the superior and inferiorendplates together in the expansion direction as the device 10 isintroduced into the intradiscal space to be distracted.

As shown particularly in FIGS. 1-3 and 6-7, the superior endplate 12includes a graft chamber defined by an opening 32 extending through theupper outer surface 12 a and the lower surface 26. In a particulararrangement, the opening 32 is situated to lie more adjacent to theproximal surface 20 or rear end of the device 10. In accordance with onearrangement, the superior endplate 12 is formed of a biocompatiblepolymer such as polyethylethylketone (PEEK). PEEK is used in fusionapplications for its combination of strength, biocompatibility, andelasticity which is similar to human bone. Other composites may includederivatives of PEEK such as carbon fiber reinforced PEEK and PEKK,respectively. In a particular aspect, the superior endplate 12 mayfurther include an upper endcap 34 that defines the outer surface 12 a.Endcap 34 may be a separate plate formed of material for the promotionof bone growth, such as titanium, and may be attached to the endplate 12with suitable conventional techniques. As an alternative, the uppersurface 12 a may be defined by a coating a suitable layer of bone growthpromotion material, such as titanium, which may be deposited byconventional techniques such as, for example, by ion implantation asdescribed in U.S. Pat. No. 4,743,493, entitled “Ion Implantation ofPlastics”, issued on May 10, 1988 to Sioshansi et al., the contents ofwhich are incorporated by reference herein.

The inferior endplate 14 of the interbody fusion device 10 as shown inFIGS. 1-3 and 6-7 is elongate and comprises a pair of opposing spacedapart sidewalls 36 and 38 extending along the longitudinal direction andprojecting upwardly from the lower outer surface 14 a. A pair of spacedapart endwalls 40 and 42 extend laterally across the device and projectupwardly from outer surface 14 a. Rear end wall 40 is disposed at therear or proximal end of the device 10 and front end wall 42 is disposedat the front or distal end of the device 10. The side walls 36, 38together with rear end wall 40 and front end wall 42 form an open,upwardly facing fully bounded interior cavity 44 as shown in FIGS. 1-2and 7. The interior cavity 44 is sized and configured to receive thesuperior endplate 12 including the hub 16 and the endcap 34 inrelatively close fit between the side walls 36 and 38 and the end walls40 and 42 of the inferior endplate 14 in a non-expanded condition asshown in FIGS. 1 and 2. The hub 16 of superior endplate 12 remains fullycontained within the inferior endplate 14 during telescoping expansionof the device 10 as shown in FIGS. 6 and 7, contributing to thetorsional strength of the expanded device 10.

The inferior plate 14 as shown in FIG. 3 defines a fully bounded waferchannel 46 extending through the rear endwall 40 in communication withinterior cavity 44 and through which the wafers 100 which serve asexpansion members are introduced. The inferior endplate 14 includes apair of opposite ledges 48 that define an upper support surface on whicheach wafer 100 is supported as it introduced into the wafer channel 46,as will be described. The ledges 48 define the bottom surface of thecavity 44. Wafers are introduced sequentially into wafer channel 46, aswill be described. The rear endwall 40 further defines a threadedconnection opening 50 for threaded releasable receipt of a guide pin foruse in the introduction of wafers 100 and in the delivery of bone graftmaterial into the device 10, as will also be described. Rear endwall 40may also additionally include a pair of bilateral notches 52 adjacentthe sidewalls 36 and 38 for use in attachment to portions of the waferinserter for the establishment of a rigid connection to the device 10for insertion into the intradiscal space.

As shown particularly in FIGS. 1-3 and 6-7, the inferior endplate 14includes a graft chamber defined by an opening 54 extending through thelower outer surface 14 a and the upper support surface 48 incommunication with cavity 44. In a particular arrangement, the opening54 is situated to lie more adjacent to the proximal surface 20 or rearend of the device 10 and at least in partial alignment with the opening32 in superior endplate 12. In accordance with one arrangement, theinferior endplate 12 is formed of a material different from the materialof the superior endplate 12. In this aspect, the inferior endplate 12may be formed of a biocompatible metal, such as titanium, for itsstrength properties. Titanium is chosen for strength, biocompatibility,processing capability, and fluoroscopic imaging properties(radiolucency). Other alternative materials include cobalt chrome,stainless steel (both stronger than titanium but much less radiolucent),or biocompatible ceramics such as silicon nitride or zirconia, which areradiolucent. Titanium and silicon nitride have demonstrated goodapposition to bone and superior to PEEK. In this regard where inferiorendplate 14 is formed of titanium, the lower outer surface 14 a wouldprovide for the promotion of bone growth. Where inferior endplate 14 isnot formed of a bone growth promotion material, lower outer surface 14 amay be coated with a suitable layer of bone growth promotion material,such as titanium, and deposited in a conventional manner as describedhereinabove.

Where inferior endplate 14 is formed of titanium or other suitable metalthat is radiopaque, windows 56 may be formed through sidewalls 36 and 38and/or through front endwall 42 as shown in FIGS. 1-3 and 6-7 so as toallow visual observation of the expansion of the device 10 uponinsertion of the wafers 100 by suitable imaging techniques, such asfluoroscopy.

Details of an interlocking wafer 100 are shown in FIGS. 4-5. The wafer100 is elongate and has an upper surface 102 and a lower surface 104,both of which are generally planar so that the wafers can form a stablestack within the interbody fusion device 10. Wafer 100 includes atrailing rear end 106 and a leading front end 108. The rear end 106 isformed substantially in the form of a horseshoe, with a pair of spacedopposing arms 112 and 114 defining an open rearward facing generallyU-shaped opening 116. The surface 118 between the upper surface 102 andthe lower surface 104 at the base of opening 116 defines a pushingsurface, as will be described. The opening 116 at the rear end of eachwafer 100 is provided to allow bone graft material to flow into thedevice 10 through the openings 116 and into the openings 32 and 54extending through the superior endplate 12 and the inferior endplate 14,respectively.

The rear end 106 includes a downward-facing sloped surface 120 at thefree end of each arm 112 and 114 that corresponds angularly to anupward-facing surface 122 on the leading front end 108 of the wafer 100.The sloped surfaces help displace an earlier inserted wafer 100 uponintroduction of a new wafer. More specifically, when a first wafer 100 ais introduced through the wafer channel 46, resting on the ledges 48,the downward-facing sloped surface 120 thereof is lifted upon contactwith the upward-facing slope 122 of a newly inserted wafer 100 b (FIG.7). This allows the newly inserted wafer to ride along the ledges 48until it is positioned fully underneath the previous wafer as more fullydescribed in the '867 patent.

The wafer 100 includes several features for interlocking engagement tothe hub 16 and to adjacent wafers 100 in a complementary interlockingmating interface. One particular feature includes a series of lockingelements defined by resiliently deflectable prongs 124 that projectoutwardly above the upper surface 102 of the wafer 100 in the directionof expansion of device 10. A complementary series of locking surfaces126 are defined in the lower surface 104 of the wafer 100 for resilientengagement with the prongs 124 as wafers are inserted into device 10 toform a stack. It should be appreciated that the prongs 124 andassociated locking surfaces 126 may be formed on either the uppersurface or the lower surface of a wafer 100 as desired. The lowersurface 104 of each wafer 100 as shown in FIGS. 5 and 7 also defines aT-slot configuration 128 for mating with a T-bar configuration 130 onthe upper surface 102 of a successive wafer 100 as shown in FIGS. 4 and7. It should be appreciated that the respective T-bar and T-slotconfigurations may also be formed on either the upper surface or thelower surface of a wafer 100 as desired. In the illustrated arrangement,there are two prongs 124 extending generally linearly and substantiallycentrally along the elongate longitudinal direction adjacent the frontend 108 of wafer 100. The structure and function of a wafer 100 and theprongs 124 are more fully described in the '867 patent, incorporatedherein by reference.

The superior and inferior endplates 12 and 14 are configured to beinitially releasably engaged by the ribs (not shown) and the grooves 30when the device 10 is unexpanded, as shown in FIGS. 1 and 2. In thisunexpanded condition, the device 10 is attached to an inserter 200 asshown in FIGS. 8 and 9. In this stage, the hub 16 is disposed within thecavity 44 of inferior endplate 14 with the ribs (not shown) on theinterior surfaces of side walls 36, 38 engaging the grooves 30 extendingalong each side of the hub 40. The lower surface 26 of hub 16 is on orclosely adjacent to the wafer support ledges 48 in facing relationship.This engagement temporarily holds the superior and inferior endplatestogether as the device 10 is introduced into the intradiscal space to bedistracted. In this unexpanded condition the outer surface 12 a of thesuperior endplate 12 is substantially flush with the upper surfaces ofthe sidewalls 36 and 38 as illustrated in FIGS. 1 and 2. In addition toproviding strength for the device 10 as described hereinabove, suchnesting of the superior endplate 12 within inferior endplate 14 allowsfor lower height of the unexpanded device 10.

The inserter 200 as illustrated in FIGS. 8 and 9 comprises a trackassembly 202 and a handle 204 for individually sequentially inserting aplurality of wafers 100 supported linearly within the track assembly202. A source of wafers 100 is provided in a cartridge 206 supported bythe track assembly 202. A pair of opposing fingers 208 is provided atthe distal end of the track assembly 202, fingers 208 releasablyengaging the notches 52 in the rear endwall 40 for connection thereto.As depicted particularly in FIG. 9, the track assembly 202 supports anelongate guide pin 210 the distal end 210 a of which is threaded forreleasable threaded connection with threaded opening 50 in rear endwall40 of the device 10. Inserter 200 comprises an elongate driver 212 thatis translatably supported within the track assembly 202, the distal endof which is configured to enter the rearward facing opening 116 of eachwafer100 and engage the pushing surface 118. Upon actuation of thehandle and translation of the driver 212, the wafer 100 is suitablymoved through the channel 46 and into the device 10 by the force of thedistal end of the driver 212 against the pushing surface 118. Inserter200 further includes a quick disconnect member 214 which upon rotationallows the inserter 200 to be detached from the guide pin 210, therebyleaving the guide pin 210 releasably connected to the expanded device 10after suitable insertion of the desired number of wafers, as shown inFIG. 10. With the guide pin 210 attached to the device 10 at opening 50,the channel 46 extending through the rear end wall 40 of device 10 isfully exposed and may be used for the introduction of suitable bonegraft material into expanded device 10. For the introduction of a bonegraft material, the guide pin 210 may be used as a locator forsubsequent attachment to an apparatus containing such bone graftmaterial whereby such apparatus may be supported by the guide pin 210while allowing access into channel 46. Further details of the structureand operation of the inserter 200 are described in commonly assignedU.S. Pat. No. 6,997,929, entitled “Tissue Distraction Device”, andissued Feb. 14, 2006, the contents of which are incorporated byreference herein.

The manner in which the interbody fusion device 10 is expanded isillustrated in FIGS. 6-7. When the first wafer 100 is introduced, theinterlocking features on the upper surface 102 of the wafer 100 engagethe mating features 28 on the lower surface 26 of superior endplate 12lifting the superior endplate 12 upwardly within the cavity 44 betweensidewalls 36, 38 and breaking the initial releasable engagement. Whenthe first inserted wafer 100 is introduced into the device 10 therearward facing opening 116 in the wafer 100 is located to be in atleast partial alignment and communication with the openings 32 and 54extending through the superior endplate 12 and inferior endplate 14,respectively. This process continues with each successive wafer 100inserted beneath a previously inserted wafer 100 until a complete stackis formed telescopically lifting the superior endplate 12 relative tothe inferior endplate 14, as depicted in FIG. 7. As each subsequentwafer 100 is introduced, the prongs 124 lockingly engage the matinglocking surfaces 126 features on the lower surfaces of each previouslyintroduced wafer 100, with the openings 116 of each wafer 100 beingdisposed such that they are in at least partial alignment andcommunication with the openings 116 of each previously introduced wafer100. The lowermost wafer 100 is supported on the support surfaces ofledges 48 with the rearward facing opening being in direct communicationwith the channel 46 extending through rear endwall 40 of inferiorendplate 14. It should be noted that all the wafers 100 are containedwithin and constricted by the opposing side walls 36, 38 and the rearand front end walls 40, 42 so as to provide additional resistanceagainst torsional movement of the spine. The inserter 200 is releasedfrom the expanded interbody fusion device 10 upon unthreading the guidepin 210 from opening 50.

Having described the interbody fusion device 10, a suitable bone filleror bone graft to promote fusion between opposing vertebral bodies may beinserted into the expanded device 10 as well as into the intradiscalspace adjacent to device 10. With the inserter 200 used to insertinserts such as wafers 100 into device 10 having been removed from theexpanded device 10, it can be appreciated that the wafer insertionchannel 40 provides clear and unobstructed access into the expandeddevice 10 and into the reaward facing openings 116 of wafers 100,facilitating the introduction of bone graft material. A suitable graftinsertion instrument using the guide pin 210 as a locator may be used toinject bone graft under pressure into the expanded device 10. Under anappropriate pressure, such bone graft will flow through into channel andopenings 116 and into the openings 32 and 56 of superior endplate 12 andinferior endplate 14. Injection of the bone graft will continue untilthe graft is stress loaded against the endplates of the opposingvertebral bodies. In some instances, bone graft may be pre-loaded intoan unexpanded device 10 prior to insertion of the device 10 into theintradiscal disc space. Suitable bone graft materials may includeautograph bone, allograft bone, bone morphogenic protein (BMP) andxenograft and synthetic derived bone substitutes, as described forexample, in the '998 Patent. It should also be understood that amaterial with a bone fusion promoting substance, such as a spongesaturated with BMP, may be placed in the openings 32 and 54 suitablyformed to support such a sponge. This will allow the fusion promotingsubstance to be pre-loaded into device 10 and not be disrupted uponexpansion of device 10 by insertion of wafers 100 as described herein.

It is contemplated that the wafers 100 described herein, be formed of abiocompatible material that is sufficiently rigid to form a solid stackas the successive wafers are inserted into the device. Thus, in onespecific embodiment, the wafers 100 are formed of PEEK or a carbon-fiberreinforced PEEK, or similar polymeric material.

In accordance with certain specific applications, the overall length ofthe device 10 as shown in FIGS. 1 and 6, as defined by the length of theinferior endplate 14, is about 25 mm. The width of the device isapproximately 9 mm. The height of the unexpanded device 10 of FIGS. 1-2with the superior endplate 12 fully nested within the inferior endplate14 is approximately 7 mm. With the introduction of five wafers 100, eachof which has a thickness of approximately 1.0 mm, the height of device10 may be expanded from an unexpanded height of approximately 7 mm to anexpanded height of approximately 12 mm. Of course, the number of wafersmay vary depending upon the particular surgery and the initial heightmay also be different. For example, device 10 may be formed to have aninitial unexpanded height of approximately 9 mm and with the addition ofseven wafers 100, each having a thickness of 1 mm, the height of device10 may be increased to approximately 16 mm. As such, it should beappreciated that these dimensions are only illustrative and that thedimensions of the device 10 and the number of wafers 100 to be insertedand their thicknesses may vary depending upon the application.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected. For instance, as shown inFIG. 11, a device 300 embodying the features described herein may beformed to have a lordotic shape, whereby the leading front end 310intended to be placed in the anterior portion of the intradiscal spacemay have a height greater than the trailing rear end 320, intended to beplaced in the posterior portion of the intradiscal space.

What is claimed is;
 1. An expandable interbody fusion device forimplantation into an intradiscal space between two opposing vertebralbodies of a spine, comprising: a first endplate having an outer surfaceconfigured to contact one of said vertebral bodies and an openingextending through said outer surface; a second endplate having an outersurface configured to contact the other vertebral body and an innersupport surface, said second endplate having opposing spaced apartsidewalls and opposing spaced apart front and rear endwalls definingtherewithin a fully bounded interior cavity, said support surface beingwithin said interior cavity, said first endplate being received withinsaid interior cavity, said second endplate having at least one openingextending through said outer surface and said support surface andcommunicating with said interior cavity, said rear endwall definingtherethrough a fully bounded channel in communication with said interiorcavity, said first endplate being movable in an expansion directionrelative to said second endplate toward the opposing vertebral body; atleast one elongate expansion member having a front end and a rear endconfigured to be introduced through said channel into said interiorcavity in an insertion direction that is substantially perpendicular tosaid expansion direction and supported by said support surfacetherewithin with the front end of said expansion member adjacent thefront endwall of said second endplate and the rear end of said expansionmember adjacent the rear endwall of said second endplate, said expansionmember having an open rearward facing opening extending through therearmost surface of said rear end and communicating with said channeland in at least partial alignment with said openings through said firstendplate and said second endplate; and cooperating locking surfaces onsaid expansion member and one of said first endplate and said secondendplate member to lock said expansion member in said device.
 2. Thedevice of claim 1, wherein said first endplate and said second endplateare formed of different materials.
 3. The device of claim 2, wherein thematerial of said second endplate is substantially radiopaque and saidsecond endplate has at least one window therethrough for observation ofone of said first endplate and said at least one expansion member duringexpansion of said device.
 4. The device of claim 1, wherein said firstendplate is formed of a biocompatible polymer and said second endplateis formed of a biocompatible metal.
 5. The device of claim 4, whereinsaid first endplate is formed of PEEK and said second endplate is formedof titanium.
 6. The device of claim 5, wherein the outer surface of saidfirst endplate and the outer surface of said second endplate eachcomprise material for the promotion of bone growth during fusion.
 7. Thedevice of claim 6, wherein said outer surface of said first endplatefurther comprises a separate endcap formed of titanium and attached tosaid first endplate.
 8. The device of claim 6, wherein said outersurface of at least one of said first endplate and said second endplatecomprises a coating adhered thereto.
 9. The device of claim 1, whereinsaid cooperating locking surfaces comprise at least one interlockingelement that is resiliently flexible in the expansion direction.
 10. Thedevice of claim 9, wherein said at least one expansion member comprisesan upper surface and a lower surface and wherein said interlockingelement comprises a resiliently deflectable prong projecting outwardlyfrom one of said upper surface and said lower surface.
 11. The device ofclaim 10, including at least two elongate expansion members ofsubstantially identical configuration.
 12. The device of claim 11,wherein said at least two elongate expansion members upon introductioninto said device reside within said interior cavity.
 13. The device ofclaim 12, wherein each expansion member includes a locking surfaceextending into the other of said upper surface and said lower surfacefrom which said prong projects, a prong on one expansion member engagingthe locking surface of the other expansion member.
 14. The device ofclaim 13, wherein each expansion member includes a plurality of prongsand locking surfaces adjacent the front end of each expansion member.15. The device of claim 1, wherein said second endplate includes asurface for attachment to an inserter for inserting said expansionmember into said device between said first endplate and support surfaceof said second endplate.
 16. The device of claim 15, wherein saidattachment surface includes a connection surface on said rear endwall.17. An expandable interbody fusion device for implantation into anintradiscal space between two opposing vertebral bodies of a spine,comprising: a first endplate having an outer surface configured tocontact one of said vertebral bodies and an opening extending throughsaid outer surface and said lower surface; a second endplate having anouter surface configured to contact the other vertebral body and aninner support surface, said second endplate having opposing spaced apartsidewalls and opposing spaced apart front and rear endwalls definingtherewithin an interior cavity, said support surface being within saidinterior cavity, said first endplate being received within said interiorcavity, said second endplate having at least one opening extendingthrough said outer surface and said support surface and communicatingwith said interior cavity, said rear endwall defining therethrough achannel in communication with said interior cavity, said second endplatebeing movable in an expansion direction relative to said first endplatetoward the opposing vertebral body; a plurality of elongate expansionmembers each having a front end and a rear end and configured to beintroduced through said channel into said interior cavity one below theother in an insertion direction that is substantially perpendicular tosaid expansion direction and supported by said support surfacetherewithin with the front end of each of said expansion membersadjacent the front endwall of said second endplate and the rear end ofeach of said expansion members adjacent the rear endwall of said secondendplate, each of said expansion members having an open rearward facingopening extending through the rearmost surface of said rear end andcommunicating with each other upon introduction and in at least partialalignment with said openings through said first endplate and said secondendplate, the rearward facing opening of at least said lowermostexpansion member being in direct communication with said channel. 18.The device of claim 17, further including cooperating locking surfaceson said expansion members and one of said first endplate and said secondendplate member to lock said expansion members in said device.
 19. Thedevice of claim 18, wherein each of said expansion members comprises anupper surface and a lower surface and wherein said cooperating lockingsurfaces further include a T-bar configuration on one of said uppersurface and said lower surface and a T-bar configuration on the other ofsaid upper surface and said lower surface to slidingly receive saidT-bar configuration upon introduction of said expansion members.
 20. Thedevice of claim 19, wherein each expansion member includes a pluralityof prongs and locking surfaces adjacent the front end of each expansionmember so that upon sliding contact of one expansion member lengthwiserelative to another expansion member, said prongs on said one expansionmember progressively engage a locking surface on the other expansionmember.
 21. The device of claim 17, wherein said channel is fullybounded by said rear endwall.
 22. The device of claim 17, wherein saidinterior cavity is fully bounded by said opposing spaced apart sidewallsand said opposing spaced apart front and rear endwalls.
 23. Anexpandable interbody fusion device for implantation into an intradiscalspace between two opposing vertebral bodies of a spine, comprising: afirst endplate having an outer surface configured to contact one of saidvertebral bodies and an opening extending through said outer surface; asecond endplate having an outer surface configured to contact the othervertebral body and an inner support surface, said second endplate havingopposing spaced apart sidewalls and opposing spaced apart front and rearendwalls defining therewithin a fully bounded interior cavity, saidsupport surface being within said interior cavity, said first endplatebeing received within said interior cavity, said second endplate havingat least one opening extending through said outer surface and saidsupport surface and communicating with said interior cavity, said rearendwall defining therethrough a fully bounded channel in communicationwith said interior cavity, said first endplate being movable in anexpansion direction relative to said second endplate toward the opposingvertebral body; at least one insert having an upper surface, an oppositelower surface, a front end, a rear end and an open rearward facingopening extending through said upper surface, said lower surface and therearmost surface of said rear end, said at least one insert beingconfigured to be introduced between said first endplate and said secondendplate through said channel into said interior cavity in an insertiondirection that is substantially perpendicular to said expansiondirection and supported by said support surface therewithin with thefront end of said insert adjacent the front endwall of said secondendplate and the rear end of said insert adjacent the rear endwall ofsaid second endplate, said rearward facing opening communicating withsaid channel and being in at least partial alignment with said openingsthrough said first endplate and said second endplate; and cooperatinglocking surfaces on said at least one insert and one of said firstendplate and said second endplate member to lock said at least oneinsert in said device.
 24. The device of claim 23, wherein said at leastone insert includes a pushing surface between said upper and lowersurface within and communicating with said open rearward facing opening.25. The device of claim 24, wherein a locking surface of saidcooperating locking surfaces is disposed on one of said upper surfaceand said lower surface adjacent the front end of said insert, andwherein said open rearward facing opening defines a U-shaped opening,the base of said opening defining said pushing surface.
 26. The deviceof claim 25, wherein said locking surface includes at least oneresiliently interlocking element.
 27. The device of claim 26, whereinsaid cooperating locking surfaces further include a T-bar configurationon one of said upper surface and said lower surface and a T-barconfiguration on the other of said upper surface and said lower surface.28. The device of claim 27, including at least two of said at least oneinsert of substantially identical configuration.
 29. The device of claim23, wherein the material of said second endplate is substantiallyradiopaque and said second endplate has at least one window therethroughfor observation of one of said first endplate and said at least oneinsert during expansion of said device.
 30. The device of claim 23,further including a threaded opening through the rear endwall of saidsecond endplate, said threaded opening being spaced from said channeland configured for threaded receipt of a guide pin of an inserter forinserting said device into the intradiscal space between said twoopposing vertebral bodies.